A mass spectrometry approach for the detection and identification of variants of theplasma protein transthyretin (TTR) is being developed. The single amino acid substitutions found in TTR are closely associated with familial transthyretin amyloidosis (ATTR), a hereditary degenerative disease. A definitive diagnosis of ATTR relies on the detection and identification of TTRvariants. The approach being developed here is based on isolation of serum TTR using immunoprecipitation. The detection of the variant is achieved by mass measurement of the intact protein with electrospray mass spectrometry (ESIN4S). The liquid chromatography/electrospray ionization mass spectrometry (LC/ESIMS) analysis of the tryptic digest of the protein followed by subsequent matrix-assisted laser desorption ionization time-of-flight mass spectrometry (NLkLDI-TOFMS) and NLALDI post source decay (PSDMS) of the relevant recovered chromatographic fraction containing the variant peptide allow the identification of unknown variants. The method wassuccessfully tested using serum from ATTR patients with known variants (Va130-4Met and Va1122-->Ile). A new TTR variant, Ser23->Asn, was detected and identified using the above method where isoelectric focusing and restriction enzyme analysis failed to identify the nature of thevariant.